Radiation-emitting semiconductor components, such as for example InGaN-based top-down mounted luminescence diodes or thin-film luminescence diodes require highly reflective mirror materials which reflect radiation emitted from the active zone toward the component rear side back toward the front side or toward the component flanks.
In the case of top-down mounted luminescence diodes, the radiation-generating epitaxial layer sequence faces toward the mounting side, i.e. the component radiates through the growth substrate if still present. In the case of thin-film luminescence diodes, the growth substrate used for the epitaxial growth of the radiation-generating epitaxial layer sequence is at least partially removed and the epitaxial layer sequence is located on a carrier substrate applied subsequently.
Furthermore, for luminescence diodes based on nitride III–V compound semiconductor material, in particular based on GaN, such as AlGaN, InGaN and InGaAlN, and also GaN itself, the mirror materials are to form an ohmic contact with the p-doped layer of the layer structure.
The problem in this context is that metals of good reflective properties in the blue spectral region, such as aluminum, do not form an ohmic contact on p-GaN or related materials, such as p-AlGaN, p-InGaN and p-InGaAlN. On the other hand, materials which form a good contact on p-GaN, such as for example platinum or palladium, have an adsorbing action in the blue spectral region and are therefore not suitable for use as mirror material. Only silver is both sufficiently reflective and suitable for the contact-connection of p-GaN, etc. However, the drawback in this case is that the mechanical stability of silver layers is insufficient for use in luminescence diodes.
The group of radiation-emitting components based on nitride III–V compound semiconductor material in the present case include in particular chips in which the epitaxially produced semiconductor layer, which typically includes a layer sequence composed of various individual layers, includes at least one individual layer which comprises a material from the nitride III–V compound semiconductor material system InxAlyGa1-x-yN, with 0≦x≦1, 0≦y≦1 and x+y≦1. The semiconductor layer may, for example, have a conventional pn junction, a double heterostructure, a single quantum well structure (SQW structure) or a multiple quantum well structure (MQW structure). Structures of this type are known to the person skilled in the art and are therefore not explained in more detail at this point.
The choice of mirror material is also difficult in the case of short-wave thin-film luminescence diodes based on InGaAlP. Gold, which is often used as mirror material at present, limits the efficiency of these diodes, on account of its relatively low reflectivity. Silver, which is more suitable in terms of reflectivity, has not hitherto been used, on account of its poor bonding and on account of migration problems.
One approach aimed at eliminating these difficulties consists in using aluminum mirrors, in which the electrical terminal is formed by a platinum layer and the optical properties are provided by the aluminum. Alternatively, it is possible to deposit silver which is fixed to a side facing away from the wafer by further metals.